The principle governing the operation of most magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance or MR). Magneto-resistance can be significantly increased by means of a structure known as a spin valve where the resistance increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment
The key elements of a spin valve are a seed layer on which is an antiferromagnetic layer whose purpose is to act as a pinning agent for a magnetically pinned layer. Next is a copper spacer layer on which is a low coercivity (free) ferromagnetic layer. When this free layer is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will be at a direction, which is dictated by the minimum energy state, determined by the crystalline and shape anisotropy, current field, coupling field and demagnetization field.
If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers suffer less scattering. Thus, the resistance in this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase.
A related effect to the GMR phenomenon described above is tunneling magnetic resistance (TMR) in which the layer that separates the free and pinned layers is a non-magnetic insulator, such as alumina or silica. Its thickness needs to be such that it will transmit a significant tunneling current.
An MTJ (magnetic tunnel junction) is readily visualized by substituting a dielectric layer for the spacer layer described above for the GMR device. The principle governing the operation of the MTJ in magnetic read sensors is the change of resistivity of the tunnel junction between two ferromagnetic layers when it is subjected to a bit field from magnetic media. When the magnetizations of the pinned and free layers are in opposite directions, the tunneling resistance increases due to a reduction in the tunneling probability. The change of resistance is typically 40%, which is much larger than for GMR devices.
FIG. 1 shows a schematic cross-sectional view of a typical TMR structure. Seen there are substrate 11 on which lies seed layer 12 of a material such as NiCr or Ta. Antiferromagnetic layer 13, typically IrMn or PtMn between about 20 and 200 Angstroms thick is next. Instead of a single pinned layer, a complex of three layers is commonly used. These are soft ferromagnetic layers 14 and 16 that are magnetized in mutually antiparallel directions and are separated by antiferromagnetic coupling layer 15 (of a material such as CoFe). Conventionally, layers 14 and 16 are referred to as AP2 and AP1 respectively. Tunneling layer 17 rests on AP1 and is itself coated by free layer 18. Capping layer 19 completes the structure.
For a TMR sensor, it is essential that AP1 provide a smooth surface for the barrier layer to grow on since a rough lead/barrier interface will usually cause high interlayer coupling and low breakdown voltage, especially for the thin aluminum oxide layer that is commonly used
Conventional techniques, such as a plasma treatment of AP1, introduce new problems. In particular, although such treatment does provide a topographically smoother surface for the barrier layer to grow on, it can also disrupt the atomic ordering at the interface between AP1 and the barrier layer, resulting in a decrease in the TMR ratio.
The present invention teaches how effective smoothing of the upper surface of AP1 can be achieved without any disruption of the atomic ordering at that surface.
A routine search of the prior art was performed with the following patent references being considered as being of interest:
Ishiwata et al., in “Tunneling magnetoresistance transducer and method for manufacturing the same”, U.S. Pat. No. 6,452,204 (Sep. 17, 2002), teach the inclusion of a thin nitrided layer between the free and pinned layers. Childress, et al., “Low resistance magnetic tunnel junction with bilayer or multilayer tunnel barrier”, U.S. Pat. No. 6,347,049 (Feb. 12, 2002), teach a barrier layer that is made up of two or more separately deposited insulating layers while in “Method of making a tunnel junction with a smooth interface between a pinned or free layer and a barrier layer”, U.S. Pat. No. 6,655,006 (Dec. 2, 2003), Pinarbasi teaches smoothing of the surface onto which the barrier layer is to be deposited, by briefly exposing it to low pressure oxygen.
The following published articles were also found to be of interest:
1) K. Ohashi et al. “Low-Resistance Tunnel Magnetoresistive Head” IEEE Transactions On Magnetics, Vol. 36, No. 5, 2000, p 2549.
2) P. P. Freitas et al., “Spin-dependent Tunnel Junctions for Memory and Read-Head applications”, IEEE Transactions On Magnetics, Vol. 36, No. 5, 2000, p 2796.
3) Dian Song et al., “Demonstrating a Tunneling Magneto-Resistive Read Head”, IEEE Transactions On Magnetics, Vol. 36, No. 5, 2000, p 2545.